Generally, a pollutant load overflowing in a combined sewer system in the event of rain is referred to as combined sewer overflows (CSO).
Here, the CSO of polluted substances according to initial rainwater is larger than an amount of polluted substances generated by outflow water from a sewage treatment plant. That is, in case of a city region, if the rainfall occurs, the polluted substances on a road and in a storm-water sewer are introduced into the sewage treatment plant at the same time due to an increase in a flow rate caused by the rainfall.
Further, when the rainfall occurs, a biochemical oxygen demand (BOD) of the polluted substances introduced into the sewage treatment plant is 150 to 300 mg/L, and a concentration of suspended solids (SS) is 250 to 400 mg/L, and a case in which the concentration of the pollutants is higher than the BOD of sewage occurs frequently.
In Korea, to reduce the amount of the polluted substances generated by the initial rainwater, the BOD of water pollutants is controlled to be 40 mg/L or less and then discharged according to sewage treatment plant standards. To improve a water quality of rivers and lakes, a method of effectively removing the polluted substances generated by the initial rainwater is required.
Here, as illustrated in FIG. 1, an existing wastewater treatment facility includes a primary depositing reservoir, a biological treatment facility, and a secondary depositing reservoir.
And in a dry season, a design flow 1Q is introduced and treated in the wastewater treatment facility.
In the existing wastewater treatment facility as described above, when the rainfall occurs, the wastewater and the rainwater are mixed at a combined sewer pipe, and a large quantity of the initial rainwater overflow is introduced therein.
In the case of the initial rainwater overflow, there is a problem that the polluted substances on the road and in the storm-water sewer are washed and introduced at the same time, and thus the BOD of the polluted substances is considerably increased.
As illustrated in FIG. 2, to solve the problem, it is defined that, when the rainfall occurs, 3Q which is three times the design flow 1Q is introduced and treated by depositing, and then 1Q is treated through a biological treatment, and 2Q is discharged. However, in the sewage treatment plant standards, a filtration rate (a surface loading rate, Q/A), which is a design standard of the primary depositing reservoir, is defined as a value of 25 to 40 m2/m3/d, and thus the introduction of the 3Q exceeds a facility standard. Therefore, since the primary depositing is not performed, a large amount of solids are introduced into a bioreactor, and thus a treatment efficiency of the wastewater treatment facility is rapidly reduced.
Meanwhile, as illustrated in FIG. 3, to solve the above-mentioned problems, there has been proposed a method in which a separate initial rainwater facility is provided at the existing wastewater treatment facility.
Here, if the initial rainwater facility such as a high-speed filtration facility is provided at the existing wastewater treatment facility, when the rainfall occurs, 3Q which is three times the design flow 1Q is introduced, and 2Q of the 3Q is transferred to and treated in the high-speed filtration facility, and the design flow 1Q introduced into the primary depositing reservoir is treated by a biological treatment facility and then discharged through the secondary depositing reservoir.
As described above, an initial rainwater treatment efficiency may be enhanced by providing the initial rainwater facility such as the high-speed filtration facility at the existing wastewater treatment facility. However, since the initial rainwater facility should be separately provided, construction costs and installation costs occur, and thus it is impossible to introduce the separate facility and apparatus due to the problems of high construction costs and securement of a separate site.
Meanwhile, to solve the problems, in Korean Patent Application No. 10-2008-0047645, there has been proposed a wastewater treatment apparatus for effectively removing polluted substances due to initial rainwater, a method thereof, and a system thereof.
FIG. 4 is a view illustrating an operation of a water level varying partition wall when an amount of the wastewater is relatively small. As illustrated in the drawing, a primary depositing unit 60′ includes the water level varying partition wall 73 between a floating matter filter 67 and a biological filter paper 69. The water level varying partition wall 73 controls a flow rate introduced from the floating matter filter 67 to the biological filter paper 69. If head loss is increased, and a flow rate introduced to the biological filter paper 69 is reduced, a residual flow rate automatically flows over the water level varying partition wall 73, and is discharged via a discharge weir 71.
Due to such a structure as described above, part of the existing primary depositing reservoir may be modified into the high-speed filtration facility to treat the initial rainwater. However, since the primary depositing reservoir is modified into the high-speed filtration facility, there is a disadvantage that an index of the primary depositing reservoir is reduced, and the surface loading rate is increased, and thus a depositing efficiency is deteriorated. Further, since a depositing process and a filtering process should be performed at the same time, the maintenance thereof is difficult.